A fuel cell usually comprises an electrolyte and two electrodes separated by the electrolyte. In a fuel cell, a fuel, e.g. hydrogen gas, is fed to one of the two electrodes and an oxidant, e.g. oxygen gas, is fed to the other electrode and chemical energy is in this way converted into electric energy.
The electrolyte is permeable to hydrogen ions, i.e. protons, but not to reactive gases such as hydrogen gas and oxygen gas.
A fuel cell generally has a plurality of individual cells known as MEUs (Membrane Electrode Units) which each comprise an electrolyte and two electrodes separated by the electrolyte.
Electrolytes used for the fuel cell are solids such as polymer electrolyte membranes or liquids such as phosphoric acid. Recently, polymer electrolyte membranes have attracted attention as electrolytes for fuel cells. Materials used for polymer electrolyte membranes are, for example, perfluorosulfonic acid polymers or complexes of basic polymers and strong acids.
The perfluorosulfonic acid polymer generally has a perfluorohydrocatbon framework, e.g. a copolymer of tetrafluoroethylene and trifluorovinyl, and a side chain which is bound thereto and bears a sulfonic acid group, e.g. a side chain having a sulfonic acid group bound to a perfluoroalkylene group. The sulfonic acid group is able to release a hydrogen ion and thus be converted into an anion, and therefore conducts protons.
Polymer electrolyte membranes comprising complexes of basic polymers and strong
The perfluorosulfonic acid polymer generally has a perfluorohydrocarbon framework, e.g. a copolymer of tetrafluoroethylene and trifluorovinyl, and a side chain which is bound thereto and bears a sulfonic acid group, e.g. a side chain having a sulfonic acid group bound to a perfluoroalkylene group. The sulfonic acid group is able to release a hydrogen ion and thus be converted into an anion, and therefore conducts protons.
Polymer electrolyte membranes comprising complexes of basic polymers and strong acids have already been developed. Thus, WO96/13872 and the corresponding U.S. Pat. No. 5,525,436 describe a process for producing a proton-conducting polymer electrolyte membrane, in which a basic polymer, e.g. a polybenzimidazole, is dipped into a strong acid, e.g. phosphoric acid, sulfuric acid, etc.
A fuel cell in which such a polymer electrolyte membrane is used has the advantage that it can be operated at temperatures of 100° C. and above.
J. Electrochem. Soc., Volume 142, No. 7, 1995, pages L121-L123, describes doping of a polybenzimidazole with phosphoric acid.
WO97/37396 and the corresponding U.S. Pat. No. 5,716,727 describe a process for producing a polymer electrolyte membrane, in which a polybenzimidazole is dissolved in trifluoroacetic acid, the solution is then admixed with phosphoric acid and the solvent is subsequently removed.
Even when a basic polymer in itself has a satisfactory mechanical strength, impregnation of the basic polymer with a strong acid for the purpose of imparting proton conductivity can result in its mechanical strength to be decreased to an unsatisfactory level. It is therefore desirable to achieve a further improvement in the mechanical strength of the basic polymer for the purpose of using the complex of the basic polymer and the strong acid as electrolyte membrane in a fuel cell, etc.
WO 00/44816 provides starting points for improving the mechanical strength and the swelling behavior of a membrane. Here, a solution comprising a basic polymer and a bridging reagent is used for casting the membrane and bridging is subsequently carried out. These membranes, too, are still in need of improvement in respect of their mechanical strength.
Further starting points for improving the mechanical strength may be found in the German patent application No. 10110752. Here, a solution comprising a basic polymer and a bridging reagent is used for casting the membrane and bridging is subsequently carried out in the presence of a basic catalyst. These membranes, too, are still in need of improvement in terms of their fracture toughness.
Further starting points for improving the mechanical strength may likewise be found in the German patent application No. 10052242.4. Here, a solution comprising a basic polymer and a polysulfone is used for casting the membrane. These membranes, too, are still in need of improvement in respect of their fracture toughness or the swelling behavior.